This description relates to a display having a controllable gray scale circuit.
Referring to FIG. 1, in some examples, a liquid crystal display 10 includes an array 12 of pixel circuits 14 that are controlled by one or more gate drivers 16 and one or more data drivers 18. Each pixel circuit 14 includes one or more thin film transistors (TFT) 20, a storage capacitor CST 22, and a liquid crystal cell (not shown in the figure). The liquid crystal cell has an effective capacitance, represented by CLC 25. The capacitors CST 22 and CLC 25 are connected to a first node 21 and a second node 23. In some examples, the first node 21 is connected to the transistor 20, and the second node 23 is connected to a reference voltage Vcom. The TFT 20 includes a gate 24 that is connected to a gate line 26, which is connected to the gate driver 16. When the gate driver 16 drives the gate line 26 to turn on the TFT 20, the data driver 18 simultaneously drives a data line 28 with a voltage signal, which is passed to the capacitors CST 22 and CLC 25.
The first and second nodes 21 and 23 are connected to two transparent electrodes (not shown), respectively, that are positioned on two sides of the liquid crystal cell. The voltage held by the capacitors CST 22 and CLC 25 determines the voltage applied to the liquid crystal cell, which controls the amount of change in the orientations of liquid crystal molecules in the cell and determines the amount of light that can pass through the cell. The voltage on the data line 28 is sometimes referred to as a “gray scale voltage” because it determines the gray scale level shown by the pixel circuit 14.
Each pixel on the display 10 includes three sub-pixels for displaying red, green, and blue colors. Each sub-pixel includes a pixel circuit 14. By controlling the gray scale levels of the three sub-pixels, each pixel can display a wide range of colors and gray scale levels.
The relationship between the voltage applied to the liquid crystal cell and the transmittance of the cell can be non-linear. FIG. 2 is a graph that shows a curve 150 representing a relationship between the gray scale voltage V (received on the data line 28) applied to the first node 21 of the storage capacitor 22 and the transmittance of the liquid crystal cell. The curve 150 is approximately symmetrical with respect to V=Vcom (which is the reference voltage applied to the second node 23 of the capacitor 22). When the gray scale voltage is equal to Vcom (zero voltage difference across the capacitor), there is a high transmittance. When the gray-scale voltage is above Vref1 or below Vref2, the transmittance is near zero. Vref1−Vcom is approximately equal to Vcom−Vref2. The transmittance of the liquid crystal cell is affected by the absolute voltage difference applied to the liquid crystal cell, regardless of the polarity of the voltage difference (positive polarity refers to the voltage at an upper electrode being greater than the voltage at a lower electrode, and negative polarity refers to the voltage at the upper electrode being smaller than the voltage at the lower electrode). In some examples, the voltage applied to the liquid crystal cell alternates in polarity (that is, the voltage on data line 28 alternates between Vcom+ΔV and Vcom−ΔV) to reduce stress imparted on the liquid crystal cell.
The data drivers 18 (FIG. 1) receive pixel data from a display controller 30, which in turn receives image or video signals from a host device 32, such as a host computer. When the display 10 is initially powered on, leakage currents from the TFTs 20 of the pixel circuits 14 may cause the data drivers 18 to drive the pixel circuits 14 before receiving pixel data from the display controller 30. When power is initially supplied to the data drivers 18, the initial states of different data drivers 18 may be different, because the data drivers 18 may have residual voltages associated with a previous image frame that was displayed prior to turning off the display 10. Even when the backlight module of the display is not turned on, ambient light may be reflected from the display, and the data drivers 18 may drive the pixel circuits 14 using the residual voltages causing the display 10 to show vertical gray stripes or bands for a short period of time before the controller 30 is initialized.